Ruby is the red variety of the mineral corundum, one of the hardest substances on Earth. Its red color is primarily due to trace amounts of chromium. Rubies are among the most sought-after gemstones, valued for their rarity, durability, and intense red color, often referred to as "pigeon blood" red.

Corundum that is any color other than red, is called Sapphire.

When a chromium atom replaces an occasional aluminum atom, it too loses 3 electrons to become a chromium3+ ion to maintain the charge balance of the Al2O3 crystal. However, the Cr3+ ions are larger and have electron orbitals in different directions than aluminum. The octahedral arrangement of the O2− ions is distorted, and the energy levels of the different orbitals of those Cr3+ ions are slightly altered because of the directions to the O2− ions.[6] Those energy differences correspond to absorption in the ultraviolet, violet, and yellow-green regions of the spectrum.